Battery parts such as terminals, which are typically made of lead or a lead alloy, are usually cold formed in order to produce a battery terminal that is free of voids and cracks. If lead or lead alloy battery terminals are pressure cast, air is left in the battery terminal cavity in the mold so that as the lead solidifies, the air bubbles prevent the battery terminal from cracking. That is, the air bubbles act as fillers so the lead remains distributed in a relatively uniform manner throughout the battery terminal. Unfortunately, air bubbles within the battery terminals cause the battery terminals to be rejects as the air bubbles can produce large voids in the battery terminal. In order to minimize the air bubbles in the battery terminal, a vacuum can be drawn in the battery terminal cavity mold; however, although the vacuum removes air from the mold and inhibits the forming of air bubbles in the battery terminal, the battery terminals cast with a vacuum in the battery terminal cavity oftentimes solidify in an uneven manner producing battery terminals with cracks or tears which make the battery terminals unacceptable for use. The present invention provides a method of forming a battery part during a pressure casting part through volume shrinkage of the mold during various phases of the solidification process.
In one embodiment of the invention, a battery terminal is cast which is substantially free of cracks and tears by pressure casting a lead alloy while a vacuum is being applied to the battery terminal cavity. At the moment when the lead in the battery terminal cavity reaches the liquid-to-solid transformation stage, a piston is driven into the mold to rapidly reduce the volume of the mold for solidification. By precisely controlling the time of application of an external compression force to the molten lead in the battery terminal cavity, and consequently, the time at which the volume of the battery terminal cavity is reduced, one can force the molten lead or lead alloy in the flowable state into a smaller volume where the pressure on the battery terminal cavity is maintained. By maintaining the pressure on the battery terminal cavity during the solidification process, the battery terminal can be cast in a form that is free of cracks and tears.
In another embodiment of the invention, the mold for forming the pressure cast battery part is sealed off while the molten lead is still in the molten state and before the molten lead can begin to solidify the supply of pressurized lead is shut off and at the same time the internal pressure of the molten lead is increased by driving a piston into the molten metal. This process is suited for those applications where the entire mold can withstand the higher pressures. That is, when the liquid metal is in a molten state an increase in pressure of the molten lead throughout the mold and the maintaining of the increased pressure during solidification can produce a battery part free of tears and cracks. This process allows one to obtain greater molding pressure than is available with conventional pressure casting techniques.
In another embodiment of the invention, the cast battery part is subjected to at least a partial cold forming during the volume contraction step by rapidly driving a piston into the solidified cast battery part with sufficient force to cold form a portion of the lead in the battery part to thereby produce a battery part that is free of cracks and tears. This method is more suitable for those battery parts where one does not want to subject the mold to excessively higher pressures than the die casting pressures.